iio: light: driver for Lite-On ltr390

Implements driver for the Ambient/UV Light sensor LTR390.
The driver exposes two ways of getting sensor readings:
1. Raw UV Counts directly from the sensor
2. The computed UV Index value with a percision of 2 decimal places

[NOTE] Ambient light sensing has not been implemented yet.

Driver tested on RPi Zero 2W

Datasheet: https://optoelectronics.liteon.com/upload/download/DS86-2015-0004/LTR-390UV_Final_%20DS_V1%201.pdf
Signed-off-by: Anshul Dalal <anshulusr@gmail.com>
Link: https://lore.kernel.org/r/20231208102211.413019-2-anshulusr@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>